Airborne particle detection system with thermophoretic scanning

ABSTRACT

A system for analyzing particles in an air stream includes a first heater element configured to deflect particles in an interior region of the air stream towards a peripheral wall of an air channel encompassing the air stream, a second heater element controllable to deflect the particles in a first lateral direction along the peripheral wall, and a third heater element controllable to deflect the particles in a second lateral direction along the peripheral wall. Thermal gradients in the air channel generated by the heater elements may thermophoretically force particles towards the peripheral wall in a direction perpendicular to the air stream to allow thermophoretic forcing and scanning of particles in either the first lateral direction or the second lateral direction along the peripheral wall and onto a surface of a particle detector. Systems and methods for scanning particles with thermophoretic forces are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplications 62/586,130; 62/586,134; 62/586,141; 62/586,148; each filedon Nov. 14, 2017; the entire contents of these applications are eachhereby incorporated herein by reference.

TECHNICAL FIELD

This patent disclosure relates generally to the field ofparticulate-matter detection and more specifically to air-qualitysensors and to systems and methods for determining airborne-particlecontent.

BACKGROUND

The presence of airborne and other gas-borne particulate matter (PM),alternatively referred to as aerosol particles, can contribute to poorair quality and potentially adverse health effects. These particles canpenetrate into human and animal lungs, contributing to lung disease,heart disease, cancer, and other illnesses. Such particles may beproduced by many sources, including industrial and agriculturalprocesses, fossil-fuel combustion in power plants and vehicles, fires,smoking, and other natural and manmade causes.

Airborne particles with a diameter of 2.5 microns or less (often termedPM2.5) tend to be particularly problematic. These finer sizedparticulates can remain suspended in the air for long periods of timeand can penetrate deep into the lung alveoli. Airborne particles under0.1 microns in diameter can pass through the lungs and enter the body,causing damage to other organs. Particles of intermediate sizes, such asbetween 2.5 and 10 microns (often termed PM10), although not aspotentially toxic as the smaller PM2.5 particles, are also medicallyproblematic because these can also penetrate into at least the outerportions of the lungs. In contrast, the larger sized particles, such asparticles over 10 microns in diameter, tend to be less problematic froma health perspective. This is because such larger particles do notpenetrate as deeply into the lungs and tend to settle out of the airrelatively quickly. The impact of nanoparticles in the range of 0.01microns to 0.1 microns is relatively unknown and is an active area ofstudy, although significant adverse health impacts are suspected.

Monitoring and controlling airborne particulate matter is of intenseinterest due to potentially adverse health and environmental effects.Various health, legal, government, scientific, industrial and commercialentities have considerable interest in methods of monitoring airborneand other gas-borne particulate matter. Methods that can furtherdistinguish between various sizes of particulate material areparticularly valued. Current systems for monitoring particulate mattertend to be relatively bulky, complex and expensive, which generallyrender them unsuitable for mass-market use.

Compact aerosol mass detection systems may use resonant sensors fordetermining the mass of particulate matter deposited on the surfaces ofthe sensors. Extended use of resonant sensors can result in reliabilityand lifetime concerns as particles aggregate on the resonant sensorsurfaces, an undesirable situation for devices targeting mass marketsfor air quality sensors.

BRIEF SUMMARY OF THE INVENTION

In some embodiments, the invention may be a system for analyzingparticles in an air stream includes a first heater element configured todeflect particles in an interior region of the air stream towards aperipheral wall of an air channel encompassing the air stream, a secondheater element controllable to deflect the particles in a first lateraldirection along the peripheral wall, and a third heater elementcontrollable to deflect the particles in a second lateral directionalong the peripheral wall. A first thermal gradient in the air channelgenerated by the first heater element may thermophoretically forceparticles towards the peripheral wall in a direction perpendicular tothe air stream. A second thermal gradient in the air channel generatedby the second heater element and a third thermal gradient in the airchannel generated by the third heater element may cooperate to allowthermophoretic forcing of particles in either the first lateraldirection or the second lateral direction along the peripheral wall.

The system may include a particle detector positioned on the peripheralwall of the air channel configured to collect particles deflected fromthe interior region of the air stream. The first, second and thirdheater elements may be configured to allow scanning of the deflectedparticles onto a surface of the particle detector. Scanning may spreadthe particles throughout the surface of the particle detector toincrease particle collection uniformity. Scanning may extend a lifetimeof the particle detector. The first thermal gradient may be selected todeflect particles in a selected particle size range onto a surface ofthe particle detector. The system may include a thermophoretic particleconcentrator including fourth and fifth heater elements disposed onopposing walls of the air channel and configured to cooperatively forceparticles in the air stream towards the interior region of the airstream. The thermophoretic particle concentrator may be positioned in anupstream direction of the air stream with respect to the first secondand third heater elements.

The system may include a controller that is electrically coupled to thefirst, second heater element and third heater elements. The controllermay be configured to control power to each of the first, second andthird heater elements to allow scanning of the deflected particlestowards the peripheral wall and to deflect particles in a selectedparticle size range onto a surface of a particle detector. The particlesmay be deflected with a thermal gradient generated by a combination ofthe first, second and third heater elements. Smaller particles in theair stream may be selectively deflected away from the interior regionand towards a periphery of the air stream at a different rate thanlarger particles in the air stream. An extended segment of the secondand third heater elements may be configured to retain particles in theinterior region of the air stream.

In some embodiments, the invention may be a method of analyzingparticles includes concentrating particles in an interior region of anair stream, deflecting the concentrated particles in the interior regionof the air stream towards a peripheral wall with a first heater element,and scanning the concentrated particles in a lateral direction along theperipheral wall with second and third heater elements. Concentrating theparticles in the interior region of the air stream may includeconcentrating the particles with a thermophoretic particle concentratorincluding fourth and fifth heater elements. The fourth and fifth heaterelements may be disposed on opposing walls of the air channel. Thefourth and fifth heater elements may be configured to cooperativelyforce particles in the air stream towards the interior region of the airstream. The thermophoretic particle concentrator may be positioned in anupstream direction of the air stream with respect to the first, secondand third heater elements. The particles deflected from the interiorregion of the air stream may be collected on a surface of a particledetector positioned on the peripheral wall. Power may be controlled toeach of the first, second and third heater elements to allow scanning ofthe deflected particles towards the peripheral wall. Power may becontrolled to deflect particles in a selected particle size range onto asurface of a particle detector. An airstream velocity of the air streammay be controlled to allow control of thermal gradients generated in theair channel.

In some embodiments, the invention may be system for analyzing particlesthat includes means for concentrating particles in an interior region ofan air stream, means for deflecting the concentrated particles in theinterior region of the air stream towards a peripheral wall, and meansfor scanning the concentrated particles in a lateral direction along theperipheral wall. The system may further include means for collecting theparticles deflected from the interior region of the air stream on asurface of a particle detector positioned on the peripheral wall andmeans for controlling power to allow scanning of the deflected particlestowards the peripheral wall and to deflect particles in a selectedparticle size range onto the surface of the particle detector.

In some embodiments, the invention may be a non-transitorycomputer-readable medium storing computer-readable program code to beexecuted by at least one processor for analyzing particles in an airstream includes program code having instructions configured to causeconcentrating particles in an interior region of an air stream,deflecting the concentrated particles in the interior region of the airstream towards a peripheral wall, and scanning the concentratedparticles in a lateral direction along the peripheral wall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a system for analyzingparticles in an air stream.

FIG. 2 illustrates a method of analyzing particles traversing a systemfor analyzing particles in an air stream.

FIG. 3 illustrates the elements and operation of a thermophoreticparticle concentrator.

FIG. 4 illustrates the elements and operation of a thermophoreticparticle discriminator.

FIG. 5 illustrates a method of fractionating and discriminatingconcentrated particles in an air stream.

FIG. 6 shows a plot of particle position across an air channel versusdistance along the air channel having a thermophoretic particleconcentrator and a thermophoretic particle discriminator.

FIG. 7 shows a block diagram of a system for analyzing particles in anair stream.

FIG. 8 depicts an exploded view of a system for analyzing particles inan air stream.

FIGS. 9A-9I illustrate top and cross-sectional views of variousthin-film heater elements for use in systems for analyzing particles.

FIGS. 10A-10C illustrate perspective and cross-sectional views of aresonant-based particle detector.

FIGS. 11A-11B illustrate the operation of a system for analyzingparticles in an air stream.

FIGS. 12A-12B illustrate a top view and a side view of a system foranalyzing particles in an air stream.

FIGS. 13A-13B illustrate a perspective view and a side view of a systemfor analyzing particles.

FIGS. 14A-14D illustrate top and side views of a system with arectangular air channel for analyzing particles in an air stream andoperation thereof.

FIGS. 15A-15D illustrate top and side views of a system with anexpanding air channel for analyzing particles in an air stream andoperation thereof.

FIG. 16 illustrates a top view of a system for analyzing particles in anair stream including a centrifugal particle separator stage.

FIGS. 17A-17B illustrate top and side views of a system for analyzingparticles with a widening air channel and a narrowing channel height.

FIGS. 18A-18B illustrate top and side views of a system for scanning andanalyzing particles.

FIGS. 19A-19B illustrate top and side views of a thermophoretic particledetection system for analyzing particles including multi-tapped heaterelements for scanning.

FIGS. 20A-20B show plots of heater voltages applied to various heatertaps along the length of multi-tapped thin-film heater elements andresultant changes in the temperature profile versus distance along theair channel.

FIG. 21 illustrates a block diagram of a system for analyzing particlesin an air stream.

FIG. 22 shows a block diagram of a method for analyzing particles in anair stream.

FIG. 23 shows a block diagram of a method for scanning and analyzingparticles.

DETAILED DESCRIPTION OF THE INVENTION

The techniques, methods, devices, and systems disclosed herein mayresult in smaller, simpler and lower cost airborne particle detectionand monitoring devices that allow mass-market use in homes, buildings,workplace environments, industrial facilities, indoor and outdoorenvironments, and personal air-quality monitors. Improved airborneparticle detection systems and methods may be used in a variety ofdevices including cellular phones, smartphones, laptop and tabletcomputers, thermostats, voice-activated tabletop monitors, wearabledevices such as watches and personal health monitors, air monitors forgreen buildings and home-automation systems, vehicle cabin monitoring,smoke detectors and protective devices such as face masks andeyeglasses, among other applications.

Improved air particle monitoring devices can be facilitated by systemsand methods configured to use thermophoretic forces. Such systems andmethods can produce various effects useful for such improved devices byemploying suitable thermal gradients. These effects may includeconcentrating and focusing particulate material, fractionatingparticulate material according to size, and directing various sizes ofparticulate material onto one or more suitable particle sensors such asresonant-based MEMS sensors or other devices in a controlled manner.

Thermophoretic force generally refers to the force that may be exertedon small particles such as micron and sub-micron sized particles thatare suspended in a gas or fluid media in the presence of thermalgradients. Absent thermal gradients (also referred to as “temperaturegradients” or “heat gradients”), suspended particles experience normalrandom Brownian motion. In the presence of thermal gradients, moreenergetic molecules of the gas or fluid media may impact one side of theparticle relative to the other side of the particle, producing a netforce on the particle that varies as a function of the particlediameter, temperature gradient, gas pressure, particle temperature andother variables such as the thermal conductivity and heat capacity ofthe particle. This thermophoretic force can, in turn, impart athermophoretic velocity to such particles that varies as a function ofthe thermal gradient, gas viscosity, gas density and the size andcomposition of the particles. The thermophoretic force may be used toconcentrate particles in an interior region of an air stream and toselectively deflect the particles towards suitable particle detectorsfor detection and analysis.

Modulating the power applied to one or more heater elements of athermophoretic particle detection system can modulate the thermalgradients and concomitant thermophoretic forces on particles travelingthrough the air channel, allowing particles of a particular size, sizerange or specific property to be selectively collected and detected on asurface of a particle detector. Later in time, particles of a differentsize or size range may be collected and detected, allowing thegeneration of spectrographic aerosol mass concentration information.Scanning particles onto the surface of a particle detector can result inmore uniform particle deposition across the detector surface to increasethe accuracy of particle property determination and to extend thelifetime of the particle detector. In some implementations, individualheater elements in an array may be selectively dithered or turned on andoff to achieve the desired deposition profile onto the particledetector. Heater elements in either or both of a thermophoretic particleconcentrator and a thermophoretic particle discriminator may bemodulated to scan the particles in either a lateral or a longitudinaldirection along the surfaces of one or more particle detectors.

While the embodiments disclosed herein generally refer to systems andmethods for analyzing particles in an air stream, “air”, althoughencompassing normal earth atmospheric air, can be any type of gas orfluid traversing the air channel. Particles in the air stream generallyrefer to micron or sub-micron sized particles with a plurality ofparticle sizes and composition that are suspended in the air stream andare generally distinct from the smaller gas molecules or atomscomprising the carrier gas. A micron (also referred to as “μm”) is aunit of length equal to one micrometer or one-millionth of a meter.

Similarly numbered elements in the various figures below apply tosimilar elements. While the figures are intended to be illustrative,dimensions and features of the various elements shown in the figures arenot always drawn to scale for clarity.

FIG. 1 shows a simplified block diagram of a system 100 for analyzingparticles in an air stream. System 100 includes an inlet 110, a particleconcentrator 120 and a particle discriminator 130. A set of data andcontrol signals 140 may be used for communicating, sending and receivingdata, and controlling system 100. Inlet air stream 112 may includeparticulate matter that may be concentrated by particle concentrator 120and then deflected, collected, detected and analyzed by particlediscriminator 130 and associated analytical components. Outlet airstream 118 may include air drawn from the inlet 110 absent particlescollected by particle discriminator 130.

In some implementations, system 100 may be wired or wirelessly coupledto a cell phone, smartphone, laptop, or other computerized devices, toenable one or more processors within the system 100 or computerizeddevice to process the data from system 100 and produce temporal andspatial (if multiple such systems are used) measurements of particulatematter content. In some implementations, system 100 may becommunicatively coupled to one or more processors in a data center or acloud computing environment to perform data processing of theparticulate matter data and to provide results of particulate matterdata procured from system 100.

FIG. 2 illustrates a method of analyzing particles 114 traversing asystem 200 for analyzing particles in an air stream. Particles 114 inthe inlet air stream 112 passing through the inlet 110 are drawn throughan air channel 102 (also referred to as a “flow channel”) that isencompassed by one or more channel walls 104. Without any focusing orconcentration, particles 114 generally follow an air stream 106 (alsoreferred to as a “flow stream”) in the air channel 102 along one or morestreamlines 108. Particles deflected by applied forces such asthermophoretic force or gravitational force may be deflected away from aparticular streamline 108 in the direction of the applied force.

Airborne particles 114 may have different sizes and shapes and comprisedifferent materials or aggregates of various materials. Various types ofairborne particles 114 at the inlet air stream 112 are depicted andlabeled 1 through 8, indicating larger spherical particles 1 and 5,smaller spherical particles 2 and 6, fibrous particles 3 and 7, andfractured particles 4 and 8. As these particles 114 traverse the airchannel 102, thermophoretic forces from heater elements 122 a, 122 b(also referred to as “heating elements” or “focusing heaters”) in theparticle concentrator 120 may force particles 114 away from a peripheryof the air channel 102 near the channel walls 104 and towards aninterior region 220 of the air channel 102 that may be at or near thegeometrical center of the air channel 102. Thermophoretic particleconcentrator heater elements 122 a, 122 b may be used to focus incomingparticles 114 into a tighter beam to facilitate analysis. Particles 114concentrated in an interior region 220 of air channel 102 may progresstowards the particle discriminator 130 along one or more streamlines108.

Particle discriminator 130 may include one or more heater elements 132(also referred to as a “precipitation heater” or a “deflection heater”)to deflect and collect particles on a wall 104 or particle detector 134opposite the heater element 132. Particles 114 may be deflected bythermal gradients generated by the thermophoretic particle discriminatorheater element 132 and directed towards a particle detector 134 near thewall 104 where particles in a selected particle size range may becollected on a surface 136 of the particle detector 134. The collectedparticles 116 may cause a shift in a resonant frequency of the particledetector 134, which may be calibrated to determine the amount of masscollected on the surface 136. Fractionation, collection, detection, andanalysis of particles 114 in a particular size range depend in part onsize-dependent and composition-dependent thermophoretic forces and aninitial well-collimated and pre-focused stream of incoming particles114. Uncollected particles may continue down the air channel 102 alongone or more streamlines 108 and exit via the outlet air stream 118.

FIG. 3 illustrates the elements and operation of a thermophoreticparticle concentrator 120. Thermophoretic particle concentrator 120 mayinclude at least one pair of thermophoretic particle concentrator heaterelements 122 a, 122 b positioned near a periphery of an air channel 102within the particle concentrator 120. Particle concentrator 120 mayinclude an air channel 102 with a first open end for inlet air stream112 and a second open end for outlet air stream 118 that allowsparticles 114 and air stream 106 to traverse the air channel 102. Theair channel 102 may be enclosed by one or more channel walls 104extending from at least the first open end to the second open end. Twoor more heater elements 122 a, 122 b may be positioned between the firstopen end and the second open end. The heater elements 122 a, 122 b maybe positioned near a periphery of the air channel 102 and cooperativelyconfigured to thermophoretically force particles 114 in the air stream106 away from the periphery and towards an interior region 320 of theair channel 102.

A position-dependent heat profile may be generated along the length ofthe air channel 102 to drive particles 114 towards the center of the airchannel 102, while otherwise allowing the air stream 106 to remainunperturbed. Particles 114 in the air stream 106 may bethermophoretically forced towards the interior region 320 of the airchannel when the heater elements 122 a, 122 b are heated and thermalgradients emanating from heater elements 122 a, 122 b are generated.Isothermal lines 324 a, 324 b show lines of constant temperature withinthe air stream 106. As power applied to heater elements 122 a, 122 bchanges and as the velocity of air stream 106 is adjusted, the positionand the shape of isothermal lines 324 a, 324 b may change.Thermophoretic forces 326 a, 326 b may be generated and act on particles114 as particles 114 in the air stream 106 traverse the air channel 102.Thermophoretic forces 326 a, 326 b increase as the thermal gradients inthe air channel 102 increase and act on particles 114 in the directionof most rapid thermal decrease, that is, in a direction perpendicular tothe isothermal lines 324 a, 324 b and with a magnitude proportional tothe gradient of the temperature in the air stream 106. When acted uponby thermophoretic forces 326 a, 326 b generated by the thermal gradientsin the air stream 106, particles 114 may be forced away from theperiphery and towards an interior region 320 of the air channel 102. Theparticles 114 may be deflected across streamlines 108 towards the centerof the air channel 102, increasing the concentration of particles 114 inthe interior region 320 of the air channel 102. In some implementations,the temperatures of the heater elements 122 a, 122 b may be varied tocontrol the position of the concentrated particles 114 within theinterior region 320 as the particles 114 continue downstream in the airchannel 102.

While the interior region 320 of the particle concentrator 120 (and ofthe particle discriminator 130) is generally centered on the geometriccenter of the air channel 102 and includes portions of the air channel102 in the vicinity of the geometric center, in some implementations theinterior region 320 may be centered about an axis that is offset fromthe centerline of the air channel 102. Dimensions of the interior regionare generally on the order of the width and/or length of the activesensor area of the particle detector 134, which may be 200 microns orless. As particle fractionation and collection improves with a tighterdistribution of particles in the interior region of the air stream 106,device performance improves with a tighter particle distribution. Insome implementations, the interior region may be defined as the spatialvolume where a simple majority of airborne particles 114 in the airstream 106 are concentrated. In some implementations, the interiorregion may be defined as a percentage of the channel volume, such as 1%,2%, 5%, 25% or 50% of the channel volume. In some implementations, theinterior region may include the geometric center of the air channel anda percentage of the area about the geometric center, such as 1%, 2%, 5%,25% or 50% of the cross-sectional area at a point along the centerlineof the air channel.

FIG. 4 illustrates the elements and operation of a thermophoreticparticle discriminator 130. The particle discriminator 130, sometimesreferred to as a deposition chamber or a settling chamber, includes anair channel 102 having one or more channel walls 104 that contain an airstream 106. The air channel 102 in the particle discriminator 130 mayextend from the inlet 110 through a particle concentrator 120 andthrough the particle discriminator 130. The particle discriminator 130may be fluidically coupled to the particle concentrator 120 and to theinlet 110. Particles 114 in the air channel 102 may be concentrated inan interior region of the air stream 106 within the particleconcentrator 120 and then passed via the fluidically coupled air channel102 into the particle discriminator 130.

The particle discriminator 130 may include at least one thermophoreticparticle discriminator heater element 132. Heater element 132 may beconfigured to deflect particles 114 in an interior region of the airstream 106 towards a peripheral wall 104 of an air channel 102 thatencompasses the air stream 106. A thermal gradient in the air channel102 generated by the heater element 132 may thermophoretically forceparticles 114 towards the peripheral wall 104 in a directionperpendicular to the air stream 106. Isothermal lines 434 a, 434 b showlines of constant temperature. The position and shape of isothermallines 434 a, 434 b may change as the power applied to heater element 132changes and as the velocity of air stream 106 is adjusted.

As particles 114 enter particle discriminator 130, the thermophoreticforce generated by one or more heater elements 132 in the particlediscriminator 130 may force particles 114 towards a periphery of the airstream 106 near one of the channel walls 104 of the particlediscriminator 130. Thermophoretic forces 436 a, 436 b may be generatedand act on particles 114 as particles 114 in the air stream 106 traversethe air channel 102. Thermophoretic forces 436 a, 436 b increase as thethermal gradient in the air channel 102 increases and act on particles114 in the direction of greatest decrease in temperature with respect toposition. When acted upon by thermophoretic forces 436 a, 436 bgenerated by the thermal gradients in the air stream 106, particles 114are forced away from the interior region of the air channel 102 andtowards a periphery wall 104.

The particles 114 may be deflected away from the interior region of theair channel 102 and towards a particle detector 134 positioned on theperiphery wall 104 of the air channel 102. The particle detector 134 maybe configured to collect particles 114 deflected from the interiorregion of the air stream 106. Deflected particles 114 may be collectedon a surface 136 of a particle detector 134 where the collectedparticles 116 may be detected and analyzed. Smaller particles in the airstream 106 may be selectively deflected away from the interior regionand towards a periphery of the air stream 106 at a different rate thanlarger particles in the air stream. Particles may be subject toarea-dependent viscous and drag forces and to gravitational forces thatare density and volume dependent. Heavier particles, for example, may beselectively deflected by gravitational forces at a higher rate thanlighter particles of similar size.

The power (e.g. electrical power) applied to the heater element 132 ofthe particle discriminator 130 may be controlled to generate and controlthermal gradients in the air channel 102 to deflect particles 114 in aselected particle size range onto a surface 136 of the particle detector134. The selected particle size range may include a particle size rangebetween about 0.01 microns and 0.1 microns, 0.01 microns and 0.3microns, 0.1 microns and 1.0 microns, 1.0 microns and 2.5 microns, 2.5microns and 10.0 microns, and 10.0 microns and larger, or other particlesize ranges of interest. Particles 114 not collected by the particlediscriminator 130 may continue downstream along one or more streamlines108 and through the outlet air stream 118. In some implementations, thepower applied to heater element 132 may be dithered, modulated orotherwise varied to more evenly spread the distribution of collectedparticles 116 on the surface 136 of the particle detector 134. In someimplementations, the power applied to heater element 132 may be variedsequentially in time to collect and analyze particles of a first sizeduring a first time period, then collect and analyze particles of asecond size during a second time period, and so forth.

FIG. 5 illustrates a method of fractionating and discriminatingconcentrated particles 114 in an air stream 106. A cross-sectional viewof a particle discriminator 130 is shown with an air channel 102 andchannel walls 104 encompassing the air channel 102 and the air stream106. Particles 114 from an inlet air stream 112 that are concentrated inan interior region of the air stream 106 may be deflected towards aperipheral wall 104 of the air channel 102 with one or more heaterelements 132 in the particle discriminator 130. The particles may bedeflected with a thermal gradient generated by the heater element 132,with smaller particles 552, 554 in the air stream 106 deflected awayfrom the interior region and towards a periphery of the air stream 106at a higher rate than larger particles 556, 558 in the air stream. Theamount of transverse displacement of the various particles may change inan amount that varies roughly inversely with the size of the airborneparticles. That is, smaller sized particles are generally deflected morethan larger sized particles. The trajectory of smaller particles underthe influence of thermophoretic forces is more readily altered comparedto larger particles under the same influences, allowing localizedparticle detectors to distinguish between different particle sizes. Therate of deflection may also be a function of mass, thermal conductivity,surface area, and other properties of the deflected particles. Forexample, denser particles of similar size may be selectively deflectedat a different rate than less dense particles.

A plurality of particle detectors 134 a, 134 b, 134 c may be positionedon a channel wall 104 of the particle discriminator 130 to allowcollection and detection of particles within one or more particle sizeranges. The lighter (smaller) particles may be collected by the firstparticle detector 134 a and the heavier (larger) particles may becollected by the second or third particle detectors 134 b, 134 c,allowing the particle discriminator 130 to distinguish between differentparticle sizes. The physical mechanism for such size fractionation anddiscrimination assumes some form of pre-focusing or concentration of theparticulate matter stream within the air channel 102 to prevent a givenparticle detector 134 a, 134 b, 134 c from collecting a mixture ofdifferent particle sizes from different heights or locations in thesample air stream 106. In the absence of active measures to concentratethe particles 114 into a beam, the particles 114 may be distributedthroughout the entire vertical profile of air channel 102. Thus largeparticles 114 positioned near the bottom of the air channel 102 may bedeflected onto the first particle detector 134 a while smaller particles114 near the top of the air channel 102 may be deflected onto the secondparticle detector 134 b, interfering with the ability of the particlediscriminator 130 to distinguish between different particle sizes orother particle properties.

Power to the heater element 132 may be controlled to allow collection ofthe deflected particles in a particular particle size range onto asurface of one of the particle detectors 134 a, 134 b, 134 c. Asillustrated in FIG. 5, smaller particles 552′ may be collected onparticle detector 134 a; somewhat larger particles 554′ may be collectedon particle detector 134 b; larger particles 556′ may be collected onparticle detector 134 c; and the largest particles 558′ may remainuncollected and continue along the air stream 106 through the outlet airstream 118, effectively generating a cutoff point for particlecollection.

FIG. 6 shows a plot 600 of particle position across an air channelversus distance along the air channel having a thermophoretic particleconcentrator 120 and a thermophoretic particle discriminator 130.Particles in the air stream initially distributed uniformly across thechannel height enter the particle concentrator 120 and are forcedtowards the interior region of the air stream by heater elements 122 a,122 b as previously described. The concentrated particles traverse theair channel towards a particle discriminator 130. The particlediscriminator 130 is positioned in a downstream direction of the airstream with respect to the particle concentrator 120.

Under normal atmospheric pressure, smaller particles, such as particlesunder about 2.5 microns in size, are minimally affected by gravity.These particles tend to move through the air channel with minimaldeflection due to gravity. Absent any other forces, these particlesfollow almost the same path as the surrounding air in the air channel.To detect and measure suspended aerosol particles, a precipitatingthermophoretic force may be applied to the particles to deflect theparticle's trajectory enough so that the particle may be propelledtowards and collected on the surface of one or more particle detectors134 a, 134 b, 134 c, 134 d.

The particle discriminator 130 may have at least one heater element 132and an array of particle detectors 134 a, 134 b, 134 c, 134 d disposedon a wall of the air channel opposite the heater element 132 that isconfigured to collect deflected and fractionated particles. Lighter andsmaller particles 632 may be deflected at a higher rate and collected byparticle detector 134 a. Heavier and larger particles 634 may bedeflected at a lower rate and collected by particle detector 134 d.Still heavier and larger particles 636 may remain uncollected and may bepassed along the air channel where they may be deflected downwards bygravitational forces acting on the particles.

Particles smaller than a particular cutoff size, such as 2.5 microns indiameter, may be selectively collected and analyzed by the particlediscriminator 130 and particles larger than the cutoff size may continuepast the particle discriminator 130. Heavier and larger particles 636above the cutoff point that are near the periphery of the air channelmay experience a lower local velocity of air than that of particles nearthe center of the air channel, allowing gravitational forces more timeto act on the heavier particles. The effect of gravity and theorientation of the particle discriminator 130 can affect the cutoffsize.

Because the thermophoretic force can be made much greater than the forceof gravity, at least for particles sized less than about 10 microns, theparticle sensors may be configured on nearly any surface of the airchannel including (relative to the force of gravity) the floor, theceiling, or the sides of the air channel. Although in this disclosurefloor and ceiling mounted particle detectors are mainly discussed, thisis not intended to be limiting. Given the comparatively large impactthat precipitation heaters can have on particle trajectories, particlesensors can also be mounted on the sides of an air channel (relative tothe force of gravity) or upside down as desired.

In some implementations, a thermophoretic particle detection system maybe configured with one or more additional stages of particleconcentrators 120 and particle discriminators 130 positioned downstreamof a first stage. Thermophoretic forces act on smaller particles anddeflect the smaller particles at a faster rate than larger particles inthe air stream. The first stage allows concentration of particles in theair stream and collection of particles in a first particle size rangewhile passing particles outside of the first range. A second stageallows re-concentrating and re-focusing of particles remaining in theair stream, and subsequent collection of particles in a second rangewith a larger particle size range than the first stage. Additionalstages with pairs of particle concentrators 120 and particlediscriminators 130 may be added. Each stage may include one or morefocusing heaters in the particle concentrator 120 and one or moredeflection heaters and particle detectors in the particle discriminator130 to redirect larger particles towards the center of the air channelthat may be re-concentrated and re-deflected for additional collectionand analysis. In some implementations, multiple stages of particleconcentrators and discriminators allow further separation ofresonant-based particle detectors resulting in improved acousticisolation and less mechanical coupling between detectors for improvedsensitivity.

FIG. 7 shows a block diagram of a system 700 for analyzing particles inan air stream. System 700 includes an inlet 110, a particle concentrator120 fluidically coupled to the inlet 110, and a particle discriminator130 fluidically coupled to the particle concentrator 120. The inlet 110,particle concentrator 120 and particle discriminator 130 include an airchannel 102 extending through the inlet 110, particle concentrator 120and particle discriminator 130 for containing an air stream 106. The airchannel 102 includes one or more channel walls 104 for containing theair stream 106. One or more heater elements 122 a, 122 b may bepositioned on opposing sides of the air channel 102 within thethermophoretic particle concentrator 120. One or more heater elements132 may be positioned on a channel wall 104 of air channel 102 withinthe particle discriminator 130. One or more particle detectors 134 maybe positioned on a wall 104 of the particle discriminator 130 oppositethe heater elements 132 to collect and detect particles. The particledetectors 134 may include one or more piezoelectric layers 734 withelectrode layers 732, 736 on each side of the piezoelectric layer 734.

System 700 may include one or more controllers 150. Controller 150 mayinclude one or more processors configured to allow concentratingparticles 114 in an interior region of the air stream 106 within theparticle concentrator 120 and deflecting the concentrated particles 114in the air stream 106 within the particle discriminator 130 withgenerated and controlled thermal gradients in the air channel 102.Controller 150 may include various electronic circuits, passive devices,metal traces, electrical interconnects and wires for sending signals toand receiving signals from heater elements 122 a, 122 b, heater element132 and particle detector 134. Electrical power and ground connectionsfor controller 150 may also be provided.

Controller 150 may include circuitry to operate the particle detector134 in a resonant mode and to detect changes in the resonant frequency.The circuitry may include signal amplifiers and preamplifiers, signalconditioning circuitry such as filters, mixers, local oscillators,demodulators, phase-lock loops, counters, A-D (analog to digital)convertors, and divide-by-n circuits, and control circuitry to determinethe frequency shifts associated with mass loading from collectedparticles on the surface of the particle detector 134. The controller150 may include processing circuitry to process data from the particledetector 134 and to analyze properties of the collected particles.

Smaller particles 114 in the air stream 106 may be selectively deflectedaway from the interior region of the air channel 102 and towards aperiphery of the air stream 106 at a higher rate than larger particles114 in the air stream 106. The controller 150 may be configured to allowcontrolling the generated thermal gradient to deflect the particles in aselected particle size range onto a surface 136 of the particle detector134. The controller 150 may be configured to draw air and to control anairstream velocity of the air stream 106 in the air channel 102. Thecontroller 150 may be configured to allow collecting particles 114within a selected particle size range on the surface 136 of particledetector 134. The controller 150 may be configured to determine aneffective mass of the particles 114 collected on the surface 136 of theparticle detector 134. The controller 150 may be configured to generatean aerosol mass concentration estimate of the particles 114 within theselected particle size range and provide or send the generated aerosolmass concentration estimate via the data and control signals 140 toanother location. The controller may be configured to correct orcompensate for temperature, relative humidity, ambient pressure, andother factors. The controller may be configured to operate in thesemanners using non-transitory computer-readable medium storingcomputer-readable program code to be executed by at least one processorassociated with the controller for analyzing particles in an air streamthrough the use of associated program code including associated programinstructions.

FIG. 8 depicts an exploded view of a system 800 for analyzing particlesin an air stream 106. System 800 includes an inlet 110 having an inletport 812 with an open end 814 for an inlet air stream 112. System 800may include an outlet port 818 with an open end 816 for an outlet airstream 118. An air channel 102 may encompass the air stream 106 andextend from the open end 814 to the open end 816. The size and shape ofthe open ends 814 and 816 may vary depending on the device andapplication. For example, circular open ends 814 and 816 may have adiameter between about 30 microns and about 50 millimeters. Rectangularopen ends 814 and 816 may have a width between about 30 microns andabout 50 millimeters and a height between about 30 microns and about 50millimeters.

The thermophoretic particle concentrator 120 may have two heaterelements 122 a, 122 b positioned on opposite sides of the air channel102. A thermophoretic particle discriminator 130 may have one or moreheater elements 132 positioned on one side of the air channel 102 and aparticle detector 134 positioned on the opposite side of the air channel102. In the implementation shown in FIG. 8, the air channel 102 iscircular and the heater elements 122 a, 122 b and 132 are positionedalong a circumference of the channel wall in a direction substantiallyperpendicular to the air stream 106 flowing in the air channel 102.Particles in the air stream 106 may be thermophoretically forced towardsan interior region 820 of the air channel 102 when the heater elements122 a, 122 b are heated and thermal gradients emanating from the heaterelements 122 a, 122 b are generated. Heater elements 122 a, 122 b andheater element 132 may include one or more thin-film heater elements,resistive films, resistive segments, heater wires, or other heatertypes. For economic and packaging reasons, the same heater type may beused in either or both the particle concentrator 120 and the particlediscriminator 130, although each heater will generally operate at adifferent temperature depending on their use as a focusing heater or aprecipitation heater. The operating temperature may vary depending inpart on the shape and placement of the heater elements, the resistivityof the heater elements, and the applied power. Operating temperaturesfor heater elements in the thermophoretic particle concentrator 120 aretypically between about 20 degrees centigrade and 50 degrees centigradeabove ambient temperature. Operating temperatures for heater elements inthe thermophoretic particle discriminator 130 are generally higher andare typically between about 50 degrees centigrade and 200 degreescentigrade above ambient temperature for effective control of particlemovement. The temperature of the heater elements 122 a, 122 b and 132and the thermal gradients generated therefrom may be controlled bycontrolling the electrical power applied to each of the heater elements,such as by controlling the amount of electrical current passed throughthe heater elements or by controlling the voltage applied across theterminals of the heater elements.

One or more banded heater elements 822 a, 822 b may be positioned on oraround portions of the inlet port 812. The banded heater elements 822 a,822 b allow circular or rectangular inlet ports 812 to be surroundedwith heater elements that extend around the entire inlet wall 104. Thebanded heater elements 822 a, 822 b may be configured with heatersegments disposed on opposite sides of the air channel 102 thatencompasses the air stream 106. The power (e.g. electrical current)applied to inlet heater elements 822 a, 822 b may generatethermophoretic forces acting on particles in the incident air stream106, forcing the particles away from the walls 104 of the air channel102 towards an interior region 820 of the air stream 106 and beginningthe particle concentration process. Further concentration of particlesin the air stream 106 may occur in the thermophoretic particleconcentrator 120 downstream of the inlet air stream 112. Temperaturesgenerated by the banded heater elements 822 a, 822 b may be as low as afew degrees above ambient temperature to deter particles from collectingon surfaces of the inlet port 812. The banded heater elements 822 a, 822b may be configured to control or limit the level of moisture orhumidity in the air channel 102 during use.

Data and control signals 140 may be communicated to and from system 800with one or more electrical connectors and signal lines coupled tosystem 800. The air channel 102 includes one or more channel walls 104.The sides of the air channel 102 will normally be surrounded by walls104, except at the open ends for the inlet air stream 112 and the outletair stream 118. At least one particle detector 134 may be positionedalong the wall 104 and configured so that the airborne particles, afterbeing propelled transversely by the precipitation heat gradient producedby the precipitation heater element 132, may be detected by the particledetector 134.

While thermophoretic particle discriminators 130 are generally used asexemplary embodiments in the various descriptions contained herein,other detection devices and methods such as particle sensors thatoperate by laser light scattering are also possible and are notdisclaimed by this disclosure. The embodiments described herein mayoperate with many different types of particle sensors. MEMS-basedparticle sensors and in particular an FBAR sensor or an addressablearray of FBAR sensors are presented as a particular type of particledetector that may be used.

Air or other carrier gas may be drawn through the air channel 102 withan air movement device (not shown) generally positioned downstream ofthe outlet air stream 118 to generate the air stream 106. The airmovement device may include a pump, blower, fan, turbine, motorized airintake device, bellows pump, membrane pump, peristaltic pump, pistonpump, positive-displacement pump, rotary vane pump, Venturi device,airflow management device, or other air drawing means for moving ordrawing air through the air channel 102. In some implementations, theair velocity of the inlet air stream 112 in the system 800 may beadequate without requiring active airflow generation by using a passiveairflow mechanism such as an ambient air flow, an ambient pressure dropacross the length of the air channel, or a convective heat process.

Particulate filters (not shown) and protective screens may be usedupstream of the particle concentrator 120 to filter out particularlylarge particles and to protect the air channel 102 from accumulatingundesirable debris. In some implementations, a size-selective particlefilter or a size-selective input device may be included as part of theinlet 110 or placed upstream of the inlet 110. In some implementations,the walls 104 of the air channel 102 may be mechanically supported andmounted in a way that provides for improved thermal isolation. In someimplementations, the air channel 102 may be suspended in an aerogel thatprovides good mechanical robustness and good thermal isolation.

FIGS. 9A-9I illustrate top and cross-sectional views of variousthin-film heater elements for use in systems for analyzing particles.Thin-film heater element 900 shown in FIG. 9A includes a thin heaterlayer 910 such as a patterned layer of nickel, chrome-nickel orplatinum. Electrical bond pads 940 and 950 allow electrical connectionsto the heater layer 910 via heater contact regions 944 and 954,respectively. One or more electrical traces 946 may be used tofacilitate electrical connections between bond pad 940 and contactregion 944. Similarly, one or more electrical traces 956 may be used tofacilitate electrical connections between bond pad 950 and contactregion 954. The illustrative cross-sectional view shown in FIG. 9B takenthrough dashed lines AA′ in FIG. 9A shows heater layer 910 positionedbetween two insulating layers 912 and 916 such as layers of silicondioxide or silicon nitride. The heater layer 910 may be positioned abovea cavity region 918 defined in a heater substrate 920 to reduce powerloss through the heater substrate 920. Bond wires 942 and 952, or otherconnective wiring, may be used to make connections to bond pads 940 and950, respectively.

FIG. 9C shows a thin-film heater element 900 with heater layer 910patterned in a serpentine manner having various heater segments 922a-922 f serially connected to each other and electrically connected toelectrical bond pads 940 and 950. Alternative implementations ofthin-film heater elements include metal foil formed into electricallyconnected serpentine heater segments on polyimide film backing such asKapton tape with patterned electrodes, or other microfabricated heaterelements such as polysilicon heater elements formed on fused quartzsubstrates. In some implementations, other sources of heat such ashigh-resistivity wire, heat pipes, radiant heating or inductive heatingmay be used to generate the thermal gradients.

FIG. 9D illustrates a cross-sectional view of a thermally isolatedwall-mounted thin-film heater element 900 with a polymeric barrier layer904 that may serve as a channel wall of an air channel. The wall-mountedthin-film heater element 900 in FIG. 9D presents no structural featuresin the air channel except for the relatively smooth outer surface of thepolymeric barrier layer 904, minimizing the level of any airflowdisruptions in the air stream. A thin heater layer 910 may be formedseparately on a plastic heater substrate 920 and laminated or otherwiseattached to the barrier layer 904 with an adhesive layer 906 such as aUV-curable adhesive or epoxy. The heater layer 910 may be electricallyconnected to bond pads 940 and 950 with one or more electrical traces946 and 956, respectively. The bond pads 940 and 950 may be attachedwith anisotropic conductive film (ACF) 980 to electrical interconnectsformed on one or more interconnect layers 962, 964 through one or moreplated flex via holes 966 and dielectric layers 968 included in aflexible printed circuit board 960 (also referred to as a “flex”). Theconstruction shown with a cutout region in one of the flex layers 970generates a cavity region 918 between the heater layer 910 and theunderlying flex layer 972 that allows a higher level of thermalisolation and lower operating power to achieve the same operatingtemperature. Additional cavity regions (not shown) may be formed inunderlying flex layers 972, 974 of the flexible printed circuit board960 to achieve additional thermal isolation. Thermal isolation of thethin-film heater layer 910 may result in improved temperature control,less temperature variation, and lower operating power. The cavity region918 may be filled with an aerogel or other thermally insulating materialto provide mechanical strength in addition to thermal isolation.

FIG. 9E illustrates a top view of a multi-tapped thin-film heaterelement 900. The multi-tapped thin-film heater element 900 in FIG. 9Emay be attached to a polymeric barrier layer that in turn may serve as achannel wall of an air channel. The heater layer 910 may be disposed ona plastic heater substrate 920 and electrically connected to bond pads940, 950 via the contact regions 944, 954 and electrical traces 946,956, among others. Each heater segment 922 g through 922 r between twoadjacent heater taps may be individually controlled by the voltagesapplied across each segment to allow control of a temperature profile inan adjacent air stream 106. Voltages between adjacent heater taps canstep up or step down in voltage level as desired to control the powerapplied to the heater segment between the adjacent heater taps. Settingthe electrical potential difference to zero across any two adjacentheater taps reduces the thermal generation between the two adjacentheater taps to zero, allowing temperature zone control andflow-dependent temperature distributions along the length of themulti-tapped heater element. Multi-tapped heater elements require fewerelectrical connections compared to individually tapped heater elementsand allow closer-spaced and continuous heater segments for improvedtemperature profile control. One or more pairs of multi-tapped heaterelements may be formed on the heater substrate 920. The heater segmentsbetween any two heater taps may be formed in any one of a variety ofshapes including straight segments, curved segments, angled segments,tapered segments, serpentine segments and segments with varying widths.One or more stub heater segments 922 l, 922 r may be included on thesubstrate 920 with independent electrical access to allow additionalcontrol over the temperature profile and thermal gradients generated inthe air stream 106.

FIG. 9F through FIG. 9I show top views of various thin-film heaterelements 900 with thermally coupled heat spreaders. Heat spreaders arethermally conductive structures that heat up when nearby thermallycoupled heater elements or heater segments are heated up. The heatspreaders may heat up to temperatures that are generally less than thetemperature of the associated heater element, allowing improved controlof the temperature distribution across the air channel 102 as an airstream 106 passes by the heater elements and heat spreaders. The heatspreaders may or may not carry current and are largely passive devices.While the heat spreaders may be mechanically and electrically connectedto and in some implementations be formed from the same material as theheater elements, the heat spreaders may be fully passive devices thatare electrically isolated from the heater elements yet close enough toextract thermal energy from the heater elements and redistribute thethermal energy throughout other portions of the air channel. Thequantity and shape of the heat spreaders may vary from heater to heateror from segment to segment within the same air channel. For example, apair of triangular heat spreaders 938 a, 938 b may be thermally coupledto heater segments 922 s, 922 t positioned near a channel wall 104 of anair channel 102 to selectively heat up air or other gas in the airstream 106 flowing through the air channel 102, as shown in FIG. 9F. Anarray of spike-shaped heat spreaders 938 c, 938 d, 938 e, 938 f amongothers may be thermally coupled to heater segments 922 u, 922 v, asshown in FIG. 9G. A thermally coupled heat spreader 938 h that extendsacross the air channel 102 to heater segments 922 w, 922 x may betapered or otherwise contoured between the heater segments 922 w, 922 x,as shown in FIG. 9H. Heat spreader 938 i may extend between and overlapassociated heater segments 922 y, 922 z, as shown in FIG. 9I. Heatertaps 928 a through 928 p may provide electrical connectivity to each ofthe heater elements or heater segments shown in FIG. 9F through FIG. 9I.One or more passive metallic heat shunts (not shown) may be configuredon one or more layers of a multi-layer flexible printed circuit board toserve as a thermal load and to alter the dynamic temperature responseresulting in higher and more controlled thermal gradients in the airchannel. One or more heat sinks (not shown) may be included to maintaina desired temperature such as an ambient temperature along one or moreportions of the air channel.

Various heat spreaders, stub heaters, heat shunts, and heat sinks may becombined with one or more multi-tapped thin-film heater elements andcontrol electronics to generate the desired thermal gradients in the airchannel for focusing, concentrating, deflecting and collecting particlesin the air stream. Thermal potential wells generated in the air streamwith control of the thermal fields from the various heat spreaders, stubheaters, heat shunts, heat sinks, heater segments, and heater elementscan effectively garner and capture particles in the air stream 106 fordetection and analysis.

FIGS. 10A-10C illustrate perspective and cross-sectional views of aresonant-based particle detector 134. The resonant-based particledetector 134 may be an acoustic resonator device such as quartz crystalmicrobalance or a film bulk acoustic resonator having a surface exposedto air or gas-borne particles. Particles collected on the surface of theresonator may change the resonant frequency of the resonant device. Thechange in the resonant frequency due to the additional mass loading maybe detected electronically. The resonant device may operate in afrequency range between a few megahertz and several gigahertz, with adetectable frequency shift on the order of a few Hertz that generallyshifts downwards as mass is added.

The particle detector 134 may include one or more of a bulk acousticwave (BAW) resonator, a thin-film bulk acoustic wave resonator (FBAR), asolidly mounted resonator (SMR), a quartz crystal microbalance (QCM), awall-mounted particle detector, a time-of-flight detector, a resonantsensor, a capacitive sensor, an infrared sensor, an optical sensor, a UVsensor, or a particle mass detector. In some implementations, theparticle detector may include a one-dimensional or two-dimensional arrayof such sensors, or more than one type of particle sensor. The particledetector may be positioned on or near a wall of an air channel 102 andmay be arranged under or near a heater element 132 of a particlediscriminator 130. In some implementations, particles in a selectedparticle size range may be deflected and collected on a surface 136 ofthe particle detector 134. The particle detector 134 may be used todetermine an effective mass and other properties of the particlescollected on the surface such as an aerosol mass concentration estimate.

In the implementation of FIG. 10A, a depiction of an FBAR-based particledetector 134 is shown including bond pads 1010 a, 1010 b, 1010 cdisposed on a substrate 1020 with an FBAR 1030 suspended over a cavity1038 and a portion of the collected particles 116 on a surface 136 ofthe FBAR 1030. Bond pads 1010 a, 1010 b, 1010 c may be used to makeelectrical connections such as signal and ground to the FBAR 1030. Thecross-sectional view of the FBAR 1030 in FIG. 10B shows the collectedparticles 116 on the surface 136 of the FBAR 1030. The FBAR 1030 mayinclude a piezoelectric layer stack having a piezoelectric layer 1034,an upper electrode 1036, and a lower electrode 1032 suspended over acavity 1038 with the FBAR 1030 suspended partially over a cavity region1038 formed in the substrate 1020. The cavity 1038 may be formedunderneath the piezoelectric layer stack to improve the acousticisolation and reduce energy loss to the substrate 1020. One or moredielectric layers 1022, 1024 such as a layer of silicon dioxide orsilicon nitride may be used to provide electrical isolation for the bondpads 1010 a, 1010 b, 1010 c and various electrical traces 1046, 1056positioned between the FBAR electrodes 1032, 1036 and the bond pads 1010a, 1010 b, 1010 c. In some implementations, the electrodes 1032, 1036and electrical traces 1046, 1056 may comprise one or more layers ofaluminum or molybdenum.

FIG. 10C illustrates a cross-sectional view of an acoustically isolatedwall-mounted particle detector 134 with a polymeric barrier layer 1004that may serve as one of the channel walls of an air channel. Thewall-mounted particle detector 134 presents no structural features inthe air channel except for the relatively smooth outer surface of thepolymeric barrier layer 1004, minimizing the level of any airflowdisruptions in the air stream. The particle detector 134 may include anFBAR 1030 having a piezoelectric layer stack with a piezoelectric layer1034, an upper electrode 1036, and a lower electrode 1032 suspended overa cavity region 1038 in the substrate 1020. One or more dielectriclayers 1022 may be used to provide electrical isolation for the bondpads 1010 a, 1010 b and various electrical traces 1046, 1056 positionedbetween the FBAR electrodes 1032, 1036 and the bond pads 1010 a, 1010 b.The FBAR 1030 may be laminated or otherwise attached to the barrierlayer 1004 with an adhesive layer 1006 such as a UV-curable adhesive orepoxy. The bond pads 1010 a and 1010 b may be attached with anisotropicconductive film (ACF) 1080 to electrical interconnects formed on one ormore interconnect layers 1062, 1064 through one or more plated flex viaholes 1066 and dielectric layers 1068 included in a flexible printedcircuit board 1060. The construction, shown with a cutout region in twoof the flex layers 1070 and 1072, generates a cavity region 1018 betweenthe substrate 1020 and the underlying flex layer 1074 that allows ahigher level of mechanical and acoustic isolation for the particledetector 134. Mechanical isolation of the particle detector 134 mayresult in improved sensitivity to added mass and less acoustic andmechanical coupling to other components.

Thermophoretic particle detection systems may include one or moreflex-based wall-mounted heater elements such as that shown in FIG. 9Dand one or more flex-based wall-mounted particle detectors 134 such asthat shown in FIG. 10C. Flex-based air channels may be formed bycombining the flex-based heater elements and the flex-based particledetectors with suitable flex-based sidewalls to form a rectangular airchannel with continuous, smooth walls and surfaces through the inlet,particle concentrator and particle discriminator. For example, one ormore layers of polyimide may be combined with the multi-layer flexassemblies and be used as the polymeric barrier layer 904 and 1004 andas the side walls of the air channel for a compact, low-profile airborneparticle detector.

FIGS. 11A-11B illustrate the operation of a system 1100 for analyzingparticles in an air stream 106. FIG. 11A shows a cross-sectional view ofa system 1100 having an inlet 110, a thermophoretic particleconcentrator 120 and a thermophoretic particle discriminator 130. Aninlet air stream 112 entering an air channel 102 between walls 104 atvarious local velocities 1112 forms a local velocity profile 1114 thatcan vary across the width, height and length of the air channel 102 yetgenerally has a higher local velocity near the center of the air channel102 that diminishes to nearly zero near the walls 104 of the air channel102.

When heater elements 122 a, 122 b on opposite sides of the particleconcentrator 120 are heated, thermal gradients are generated throughoutthe air channel 102, which in turn generate thermophoretic forces 1126a, 1126 b that are perpendicular to isothermal lines 1124 a, 1124 b andpoint generally in the direction of the steepest negative thermalgradient. Particles in the air stream 106 may be directed away from aperiphery of the air channel 102 in the particle concentrator 120 andtowards an interior region of the air channel 102.

When heater elements 132 on one side of the particle discriminator 130are heated, thermal gradients are generated throughout the air channel102, which in turn generate thermophoretic forces 1136 a, 1136 b thatare perpendicular to isothermal lines 1134 a, 1134 b and point in thedirection of the steepest negative thermal gradient. Particles in theair stream 106 within the air channel 102 may be directed away from aninterior region of the air channel 102 in the particle discriminator 130towards a periphery of the air channel 102.

As shown in FIG. 11B, particles 1152, 1154, 1156 and 1158 withincreasing particle size are thermophoretically forced towards aninterior region of the air stream 106 in the particle concentrator 120and then are deflected away from the interior region of the air stream106 in the particle discriminator 130 towards a periphery of the airstream 106, with smaller particles undergoing greater deflection thanlarger particles. In FIG. 11B, smallest particle 1152′ is deflected andstrikes a wall 104 of the particle discriminator 130 before the particledetector 134; small particle 1154′ strikes and is collected on a surfaceof the particle detector 134; large particle 1156′ is not collected bythe particle detector 134 and continues in the air stream 106; andlargest particle 1158′ continues in the air stream 106 with lessdeflection than smaller particles.

FIGS. 12A-12B illustrate a top view and a side view of a system 1200 foranalyzing particles in an air stream 106. System 1200 includes an inlet110, a thermophoretic particle concentrator 120 and a thermophoreticparticle discriminator 130. The thermophoretic particle concentratorincludes an air channel 102 between channel walls 104 having a firstopen end for an inlet air stream 112 and a second open end for an outletair stream 118. The air channel 102 is enclosed by channel wall 104extending from at least the first open end to the second open end. Twoor more heater elements 122 a, 122 b may be positioned between the firstopen end and the second open end and are positioned near a periphery ofthe air channel 102. A cross-section of the air channel 102 and channelwall 104 perpendicular to the air stream 106 is rectangular, and atleast two heater elements 122 a, 122 b are positioned on two opposingsides of the channel wall 104. The heater segments 1222 a of heaterelements 122 a, 122 b extend along the channel wall 104 in a directionsubstantially parallel to the air stream 106. The particle discriminator130 including one or more heater elements 132 and particle detectors 134is coupled to the air channel 102 in a direction downstream from theparticle concentrator 120.

FIGS. 13A-13B illustrate a perspective view and a side view of a system1300 for analyzing particles. System 1300 includes an inlet 110, athermophoretic particle concentrator 120 and a particle discriminator130. Thermophoretic particle concentrator 120 includes a pair ofthermophoretic heater elements 122 a, 122 b positioned near a peripheryof an air channel 102. The thermophoretic heater elements 122 a, 122 bare configured to thermophoretically force airborne particles in the airchannel 102 away from the periphery and towards an interior region ofthe air channel 102 and air stream 106. The channel walls 104 and thecross-sectional geometry of the air channel 102 may be rectangular.Channel walls 104 include portions of a lower wall 1312, side walls1314, 1316 and upper wall 1318. The perspective view shown in FIG. 13Ahas the upper wall 1318 removed for clarity. An inlet air stream 112enters an opening in the channel walls 104 upstream of the particleconcentrator 120 and exits an opening in the channel walls 104downstream of the particle discriminator 130. The thermophoretic heaterelements 122 a, 122 b may include one or more heater wires 1322 a, 1322b, 1322 c suspended in the air channel 102 with heater posts 1326 a,1326 b. In some implementations, the heater wires 1322 a, 1322 b, 1322 cmay be formed into a wire mesh. Alternatively, heater elements 122 a,122 b may be constructed of thin, partially conductive films on theinterior surfaces of electrically insulated channel walls, ceilings, andfloors. Electrical current may be sent through heater wires 1322 a, 1322b, 1322 c to generate the desired thermal gradient.

One or more of the heater wires 1322 a, 1322 b, 1322 c of heaterelements 122 a, 122 b may be angled with respect to the air channel 102in an inward direction along the air channel 102 and towards an interiorregion of the air stream 106. The thermophoretic heater elements 122 a,122 b are configured to thermophoretically force airborne particles inthe air channel 102 away from the periphery and towards an interiorregion of the air channel 102 and air stream 106. The thermophoreticheater elements 122 a, 122 b allow focusing of particles in the inletair stream 112 into a tighter beam of particles with higher particleconcentration. Some of the dimensions of the air channel 102 in theregion of the particle concentrator 120 may be narrowed to furtherdirect the particles into a narrower beam.

System 1300 includes a particle discriminator 130 with at least oneheater element 132 and at least one particle detector 134 positioned ona lower wall 1312 downstream of the particle concentrator 120 to collectand detect particles in the air stream 106. Heater element 132 mayinclude a heater wire 1332 suspended in the air channel 102 with heaterposts 1336 a, 1336 b. The dimensions of the walls 104 within theparticle discriminator 130 may be narrowed to further concentrate theparticles and to increase the magnitude of the thermal gradient thatresults in an increase of the thermophoretic forces acting on theparticles. In some implementations, dimensions of the channel walls 104in other directions may be widened to slow the speed of airborneparticles and allow more time for the thermophoretic heater elements 132to force and deflect the particles onto the particle detector 134.

In some implementations, the three-dimensional structure of the airchannel 102 and/or the configuration of the various individuallycontrolled focusing heaters and precipitation heaters may includeindividually controlled focusing and precipitating heaters located onthe top, bottom, and sidewalls of the air channel 102 to produce a moreversatile type of airflow particle detecting device. This device may bereconfigured according to local conditions and particle monitoringobjectives. Such reconfiguration may be facilitated by use of suitableprocessors and control software to control the operation of the variousheater elements, particle detectors, and airflow management devices. Theeffective shape of the flow channels constraining the particulate mattermay be programmable to allow compensation for changes in variousenvironmental variables such as air pressure, temperature, and humidity.

FIGS. 14A-14D illustrate top and side views of a system 1400 with arectangular air channel 102 for analyzing particles in an air stream 106and operation thereof. An air stream 106 enters a rectangular airchannel 102 from an inlet air stream 112, traverses the air channel 102through a thermophoretic particle concentrator 120 and a thermophoreticparticle discriminator 130, and exits the air channel 102 through anoutlet air stream 118. Particle concentrator 120 includes a pair ofheater elements 122 a, 122 b positioned on one side of the air channel102 and another pair positioned on an opposite side of the air channel102. Each heater element 122 a, 122 b may have multiple heater segments1422 a, 1422 b, 1422 c with heater segments 1422 a and 1422 c extendingalong the channel wall 104 in a direction substantially parallel to theair stream 106 and heater segments 1422 b extending along the channelwall 104 in a direction that is angled with respect to the air stream106. Particle discriminator 130 fluidically coupled to the air channel102 includes a heater element 132 with heater segments 1432 a extendingparallel to the air stream 106 and a heater segment 1432 b extending ina direction perpendicular to the air stream 106 for concentrating anddeflecting particles 114 in the air stream 106. A plurality of particledetectors 134 a, 134 b, 134 c may be included in the particlediscriminator 130 for collecting, detecting and analyzing particles.Particles 114 traversing the air channel 102 may be concentrated in aninterior region of air stream 106 within the thermophoretic particleconcentrator 120. Particles 114 traversing the thermophoretic particlediscriminator 130 may be deflected and collected on a surface of one ofthe particle detectors 134 a, 134 b, 134 c for detection and analysis.Additional heater elements (not shown) may be positioned abovedownstream particle detectors 134 b, 134 c to allow greater control overthe generated thermal gradients in the air channel 102 and to allow thedownstream heater elements to be operated at a higher temperature thanthe upstream heater elements so that larger particles passed by thefirst particle detector 134 a may be forced onto one or more of thedownstream particle detectors 134 b, 134 c.

FIGS. 15A-15D illustrate top and side views of a system 1500 with anexpanding air channel 102 for analyzing particles 114 in an air stream106 and operation thereof. Particles 114 may enter system 1500 throughan inlet air stream 112 into an inlet 110 and traverse air channel 102through particle concentrator 120 and particle discriminator 130. Theparticles 114 may be concentrated within the particle concentrator 120when heater elements 122 a, 122 b are heated and thermal gradientsgenerated within the air channel 102 thermophoretically force theparticles 114 towards an interior region of the air channel 102. Heaterelements 122 a, 122 b contain heater segments 1522 a, 1522 b in anexpanding region 1560 of the air channel 102 within the particleconcentrator 120. Heater segment 1522 b is oriented in a directionsubstantially perpendicular to the air stream 106 and cooperates withheater segments 1522 a in the expanding region 1560 to concentrate theparticles 114 in an interior region of the air stream 106, even asstreamlines 108 diverge in the expanding region 1560. Particlediscriminator 130 includes a heater element 132 with heater segments1532 a and 1532 b configured to deflect particles 114 onto one or moreparticle detectors 134 a, 134 b, 134 c. Heater segments 1532 a, 1532 bmay be arranged into a “W” configuration to form a thermal potentialwell for retaining divergent particles 114 near the center of the airchannel 102 and to deflect the particles 114 onto one or more particledetectors 134 a, 134 b, 134 c within the particle discriminator 130. Insome implementations, the heater segments 132 a, 132 b of heater element132 may be configured as a sideward “V” to corral particles in the airstream 102 and to deflect the particles onto the particle detectors 134a, 134 b, 134 c. In some implementations, the heater segments 1532 a,1532 b may comprise a plurality of serpentine segments to increase theresistance of the heater element 132 and increase the heater voltageapplied across the heater element 132.

The application of an external force such as centripetal force, can, insome implementations, be used to improve the ability to differentiateand discriminate between different particle sizes. FIG. 16 illustrates atop view of a system 1600 for analyzing particles in an air stream 106including a centrifugal particle separator stage 1660. Particles 114entering system 1600 in an inlet air stream 112 traverse inlet 110,particle concentrator 120, and centrifugal particle separator stage 1660having a curved air channel 102 positioned between the particleconcentrator 120 and a particle discriminator 130. Particles in the airstream 106 may be spatially separated with smaller, lighter particlesstaying near an inside of the air channel 102 and larger, heavierparticles moving towards an outer portion of the centrifugal particleseparator stage 1660. The particle discriminator 130 may include a one-or two-dimensional array of particle detectors 134 configured to detectspatially separated particles from the centrifugal particle separatorstage 1660. The system 1600 may further include an airflow expansionstage 1670 positioned between the centrifugal particle separator stage1660 and the particle discriminator 130. The airflow expansion stage1670 may have an air channel 102 that widens as the air stream 106traverses the airflow expansion stage 1670. Particles spatiallyseparated in the centrifugal particle separator stage 1660 may befurther separated in the airflow expansion stage 1670 as streamlineswithin the airflow expansion stage 1670 diverge. Additionally, the airchannel 102 within the airflow expansion stage 1670 may widen to slowthe air velocity and particle velocity in the air stream 106 as the airstream 106 traverses the airflow expansion stage 1670 to allow more timefor thermophoretic forces to act on and deflect the particles.

Prior to entering the centrifugal particle separator stage 1660,particles may be concentrated in an interior region that is somewhatoffset from a centerline of the air channel 102 in the centrifugalparticle separator stage 1660. The thermophoretic particle concentrator120 may have heater elements 122 a, 122 b with heater segments 1622 a,1622 b, 1622 c configured to force particles in the air stream 106towards an interior region that is offset from the centerline of airchannel 102 within the particle concentrator 120 to utilize more of theair channel 102 in the centrifugal particle separator stage 1660. Toforce particles towards an interior region offset from the centerline,heater segments 1622 a and 1622 c may be extended in a directionparallel to the air channel 102 and air stream 106 with heater segment1622 c positioned closer to a centerline of the air channel 102 in theparticle concentrator 120 and heater segment 1622 b extended in adirection that is angled with respect to the air stream 106.

FIGS. 17A-17B illustrate top and side views of a system 1700 foranalyzing particles with a widening air channel 102 and a narrowingchannel height. System 1700 includes an inlet 110, a thermophoreticparticle concentrator 120 and a thermophoretic particle discriminator130.

The thermophoretic particle concentrator 120 includes an air channel 102between channel walls 104 having a first open end for an inlet airstream 112 and a second open end for an outlet air stream 118. The airchannel 102 is enclosed by channel walls 104 extending from at least thefirst open end to the second open end. Two or more heater elements 122a, 122 b each having at least one heater segment 1722 a may bepositioned between the first open end and the second open end and near aperiphery of the air channel 102. The cross-sectional geometry of theair channel 102 and channel wall 104 perpendicular to the air stream 106is rectangular, and at least two heater elements 122 a, 122 b having atleast one heater segment 1722 a are positioned on two opposing sides ofthe channel wall 104. The heater segments 1722 a of heater elements 122a, 122 b extend along the channel wall 104 in a direction substantiallyparallel to the air stream 106.

The particle discriminator 130 including heater element 132 and particledetectors 134 a, 134 b, 134 c is coupled to the air channel 102 in adirection downstream from the particle concentrator 120. The width ofthe air channel 102 increases and the height of the air channel 102decreases in the downstream direction within the particle discriminator130, allowing the airstream velocity and particle velocity to slow andthe thermal gradient to increase in the vicinity of the particledetectors 134 compared to a cross-sectional geometry of constantdimensions. Extended heater segments 1732 a of heater element 132 extendin a direction nominally parallel to the air stream 106 and areconfigured to retain or further concentrate particles in an interiorregion of the air stream 106, even as the air channel 102 widens. Heatersegment 1732 b of heater element 132 extends in a direction nominallyperpendicular to the air stream 106 near the particle detectors 134 a,134 b, 134 c to allow deflection, collection, detection and analysis ofparticles in the air stream 106. The cross-sectional geometry of the airchannel 102 within the particle discriminator 130 such as the channelheight may be narrowed as the air stream 106 traverses the particlediscriminator 130. The narrower region of the air channel allows lessdistance between the heater segments 1732 b and the particle detectors134 a, 134 b, 134 c, which may result in a higher thermal gradient and alarger thermophoretic force to be generated on particles traversing thenarrowed region.

In operation, smaller particles 1714 a, 1714 b that are concentrated inthe particle concentrator 120 may be deflected in the particlediscriminator 130 by thermal gradients generated by heater segments 1732a, 1732 b of heater element 132 and collected on a surface of a particledetector 134 a. Larger particles 1714 c, 1714 d that are concentrated inthe particle concentrator 120 may then be deflected by thermal gradientsgenerated by heater segments 1732 a, 1732 b of heater element 132 andcollected on a surface of a particle detector 134 b or particle detector134 c. Controlling the velocity of the air stream 106 and the thermalgradients generated in the particle discriminator 130 allow forselective deflection and collection of particles in a particularparticle size range onto a surface of one of the particle detectors 134a, 134 b, 134 c.

As particulate matter collects over time on the surface of the particledetectors, the particulate matter may form non-uniform depositionprofiles, such as a wedge where one side of the deposition may bethicker than the other or an island where particles collect in alocalized region of the detector surface. Wedge-shaped non-uniformitiesin the thickness of the coated material on the surface of aresonant-based sensor may cause a premature degradation of the deviceperformance by reducing the quality factor or resonant Q of the sensor.A loss of quality factor from the non-uniform deposition of particulatematter may be related to energy trapping in the resonator, where taperedgeometries guide trapped acoustic energy out of the resonator. Islandformation may also lead to a loss in the quality factor, as mass-loadedregions of the resonator tend to vibrate at a lower frequency thannon-loaded regions. Non-uniform mass loading and Q degradation mayhamper the minimum level of detection and reduce the sensor lifetime.

FIGS. 18A-18B illustrate top and side views of a system 1800 forscanning and analyzing particles. System 1800 includes an inlet 110, aparticle concentrator 120 and a particle discriminator 130. The particlediscriminator 130 includes a heater element 132 configured to deflectparticles in an interior region of the air stream 106 towards aperipheral wall 104 of an air channel 102 encompassing the air stream106. A heater element 1832 a may be controlled to deflect the particles1814 in a first lateral direction 1840 a across the peripheral wall 104.A heater element 1832 b may be controlled to deflect the particles 1814in a second lateral direction 1840 b across the peripheral wall 104. Athermal gradient in the air channel 102 generated by heater element 132may thermophoretically force particles towards the peripheral wall 104in a direction nominally perpendicular to the air stream 106. Control ofthe heat gradient generated by heater element 132 may allow scanning ofthe particles 1814 in either longitudinal direction 1842 a orlongitudinal direction 1842 b. A thermal gradient in the air channel 102generated by heater element 1832 a and a thermal gradient in the airchannel 102 generated by heater element 1832 b may cooperate to allowthermophoretic forcing of particles in either the lateral direction 1840a or the lateral direction 1840 b along the peripheral wall 104. Thermalgradients generated by a combination of heater elements 132, 1832 a and1832 b may cooperate to allow deflection, scanning and collection ofparticles 1814 in the air stream 106 onto a surface of the particledetector 134 in either lateral directions 1840 a, 1840 b, longitudinaldirections 1842 a, 1842 b, or a combination thereof. Throughout extendedoperation of system 1800, collected particles 1816 on a surface 1836 ofthe particle detector 134 with controlled scanning may be spread out onthe detector surface 1836 to increase particle collection uniformity,retain a higher quality factor of the resonant sensor, increase accuracyof particle property measurements, and extend the operating lifetime ofthe particle detector 134. In some implementations, the scanning may beadjusted to overshoot particles past the edges of the active sensor areato achieve improved edge-to-edge uniformity.

A particle detector 134 positioned on the peripheral wall 104 of the airchannel 102 may be configured to collect particles 1814 deflected fromthe interior region of the air stream 106. Heater elements 132, 1832 aand 1832 b may be configured to allow scanning of the deflectedparticles 1814 onto a surface of the particle detector 134. The scanningmay spread the particles throughout the surface of the particle detector134 to increase particle collection uniformity and extend a lifetime ofthe particle detector 134. In some implementations, the thermalgradients may be selected to deflect particles 1814 in a selectedparticle size range onto a surface of the particle detector 134.

A thermophoretic particle concentrator 120 fluidically coupled to theparticle discriminator 130 may include heater element 122 a and heaterelement 122 b disposed on opposing walls 104 of the air channel 102.Heater element 122 a and heater element 122 b may be configured tocooperatively force particles 1814 in the air stream 106 towards theinterior region of the air stream 106. The thermophoretic particleconcentrator 120 may be positioned in an upstream direction of the airstream 106 with respect to heater elements 132, 1832 a and 1832 b of thethermophoretic particle discriminator 130.

System 1800 may include a controller electrically coupled to heaterelements 132, 1832 a and 1832 b and configured to control power to eachof the heater elements 132, 1832 a and 1832 b to allow scanning of thedeflected particles 1814 towards the peripheral wall 104 and to deflectparticles 1814 in a selected particle size range onto a surface of aparticle detector 134. The particles 1814 may be deflected with athermal gradient generated by a combination of heater elements 132, 1832a and 1832 b. Smaller particles in the air stream may be selectivelydeflected away from the interior region and towards a periphery of theair stream 106 at a different rate than larger particles in the airstream. The controller may be electrically coupled to heater elements122 a, 122 b in the particle concentrator 120 to concentrate theparticles 1814 in air channel 102.

FIGS. 19A-19B illustrate top and side views of a thermophoretic particledetection system 1900 for analyzing particles 114 including multi-tappedheater elements 122 a, 122 b for scanning. An air channel 102 enclosedby channel walls 104 extends through the inlet 110, particleconcentrator 120 and particle discriminator 130. The multi-tapped heaterelements 122 a, 122 b have heater segments 1922 a, 1922 b, 1922 cextending through the particle concentrator 120 and additional heatersegments 1922 d extending through the particle discriminator 130. Stubheater segments 1922 e may be provided to allow additional control overthe temperature profile and thermal gradients generated in the airchannel 102 and air stream 106. Heater segments 1922 d extend nominallyparallel to the air stream 106 in the particle discriminator 130 whereasheater segments 1932 a, 1932 b, 1932 c extend in a direction nominallyperpendicular to the air stream 106 on a side opposite the particledetectors 134 a, 134 b, 134 c to allow deflection, collection,detection, and analysis of particles entering through the inlet airstream 112.

Heater taps 1928 a through 1928 n provide electrical connections to thevarious heater segments in heater elements 122 a, 122 b for selectiveapplication of electrical power to allow control over the heat generatedby each heater segment and the thermal gradients generated in the airchannel 102. Controlling the velocity of the air stream 106 and thethermal gradients generated in the particle concentrator 120 and theparticle discriminator 130 allows for selective concentration,deflection, and collection of particles 114 onto a surface of one of theparticle detectors 134 a, 134 b, 134 c. The multi-tapped heater elements122 a, 122 b allow continuous heater segments, local temperature zonecontrol, a configurable temperature profile, reduced electrical leadoutrequirements, and a flexible target particle response. In someimplementations, multi-tapped heater elements 122 a, 122 b may comprisea thermally isolated wall-mounted thin-film heater element with apolymeric barrier layer that serves as a channel wall for the airchannel 102. In some implementations, one or more heater segments ofheater elements 122 a, 122 b may be modulated with a varying voltage toallow controlled scanning of particles 114 in a longitudinal directionor a lateral direction with respect to the air stream 106 so thatparticles of a selected particle size range may be collected on one ofthe particle detectors 134 a, 134 b, 134 c. Modulation of voltagesapplied across stub heater segments 1922 e and to deflection heatersegments 1932 a, 1932 b, 1932 c may aid in particle selection,deflection, fractionation, detection and collection uniformityimprovements.

Scanning of particles in the air channel 102 may be performed bymodulation of voltages applied across one or more heater segments 1922a, 1922 b, 1922 c, 1922 d, 1922 e, 1932 a, 1932 b, 1932 c. Varying thevoltage across any of the heater segments varies the generated thermalgradients and causes the particles 114 in the air stream 106 to moveaccordingly. For example, scanning the particles laterally across theair channel 102 may be achieved by modulating the heater voltagesapplied to heater segments on one side or the other of the air channelto push the particles in one lateral direction or the other in responseto the modulated thermal gradient. Scanning the particles longitudinallyin the direction of the air stream 106 may be achieved by modulating theheater voltages applied to heater segments on the top side or bottomside of the air channel 102 to produce more or less deflection force onthe particles, resulting in an earlier or a later impact with thechannel wall and particle detectors 134 a, 134 b, 134 c attachedthereto. Scanning the particles longitudinally allows particles in aselected particle size range to be deflected towards a peripheral wall104 of the air channel 102 and onto a surface of a particle detector 134a, 134 b, 134 c where the particles may be collected and analyzed.

The service lifetime of thermophoretic particle detection system 1900may be extended by controlling and limiting the amount of particulatematter collected on the surface of the particle detectors 134 a, 134 b,134 c. For example, particle detector 134 a may be operated inconjunction with overlying deflection heater segment 1932 a to collectparticulate matter while particle detectors 134 b, 134 c and associatedheater segments 1932 b, 1932 c remain in an off condition. After aselected time period of seconds, minutes, hours or days depending on theapplication, particle detector 134 a and deflection heater segment 1932a may be turned off and particle detector 134 b with deflection heatersegment 1932 b may be turned on and put into operation. After anotherselected time period, particle detector 134 b and deflection heatersegment 1932 b may be turned off and particle detector 134 c withdeflection heater segment 1932 c may be put into operation, and soforth. In this manner, the power applied to each of the heater segments1932 a, 1932 b, 1932 c may be controlled to allow scanning of thedeflected particles toward a peripheral wall and onto a surface of theparticle detectors 134 a, 134 b, 134 c. The power applied to one or moreof the heater segments 1922 c, 1922 d or 1922 e may be controlled todeflect particles in a lateral direction along the peripheral wall. Insome implementations, the incremental change in the resonant frequencyor the shift in frequency from a baseline value for resonant-basedparticle detectors may be used to determine which pair of particledetectors and heater segments are operated and for how long.

FIGS. 20A-20B show plots 2000 a, 2000 b of heater voltages applied tovarious heater taps 2028 a through 2028 k along the length ofmulti-tapped thin-film heater elements 122 a, 122 b, and the resultantchanges in the temperature profile versus distance along the airchannel. The air channel includes a thermophoretic particle concentrator120 with multi-tapped thin-film heater elements 122 a, 122 b disposed onopposite sides of the air channel and a thermophoretic particlediscriminator 130 with heater element 132 disposed on a side oppositeparticle detectors 134 a, 134 b, 134 c. The heater elements 122 a, 122 bare configured to thermophoretically force particles in the air streamtowards the interior region of the air stream and the heater elements2032 a, 2032 b, 2032 c are configured to deflect particles away from theinterior region of the air stream and towards a peripheral wall to becollected by the particle detectors 134 a, 134 b, 134 c. The voltageapplied to each of the heater taps may be set to reach a desiredtemperature profile across the width and along the length of the airchannel to generate the thermal gradients and resulting thermophoreticforces on any particles in the air stream traversing the air channel. Asthe voltage across any two adjacent heater taps is increased, the amountof power applied to the heater segments between the two heater taps isincreased and the temperature in the air channel increases. Increasingthe voltage on any of the heater taps generally increases thetemperature near adjacent heater segments, while decreasing the voltagegenerally decreases the temperature in the air channel. The temperatureprofile and the thermal gradients in the air channel may be controlledby controlling the power applied to each of the heater segments in theheater elements and by controlling the velocity of the air stream in theair channel. The voltages on the heater segments may be modulated,dithered or otherwise controlled to deflect and scan particles in theair stream in either a lateral or a longitudinal direction along theperipheral wall.

FIG. 21 illustrates a block diagram of a system 2100 for analyzingparticles in an air stream. Particle detection system 2100 includes acontroller 2110 with one or more processors and circuitry for runningprogram code and executing instructions to analyze particles in an airstream among other functions. Controller 2110 may be connected via acommunications bus 2140 to one or more memories 2112. Memory 2112 mayinclude a combination of volatile and non-volatile memory for storingprogram instructions and data. Controller 2110 may communicate withother processors and data systems external to system 2100 via one ormore wireless communication links 2114 and antennas 2116 or one or morewired communication links 2118 and external communication lines 2120such as Ethernet or USB connections. One or more power sources 2122 andground lines 2124 such as batteries or AC/DC power connections mayprovide local regulated power for devices connected to bus 2140. Varioussensors 2126 and transducers such as temperature sensors, pressuresensors, humidity sensors, accelerometers, gyroscopes, ambient lightsensors, clocks, microphones, and speakers may be connected tocontroller 2110 via communications bus 2140.

One or more particle detection modules 2130 for detecting particles inan air stream may include an inlet air stream 112 for incoming sampleair and an outlet air stream 118 for outgoing air. The particledetection module 2130 may include one or more inlets, thermophoreticparticle concentrators, and thermophoretic particle discriminators. Theair stream within the particle detection module 2130 may be encompassedby the walls of an air channel extending from a first open end for theinlet air stream to a second open end for the outlet air stream. Theparticle detection module 2130 may be connected to controller 2110 viacommunications bus 2140 or other dedicated control and/or data lines.Controller 2110 may send control signals to control the power applied tovarious heater elements coupled to the air stream in the particledetection module 2130. Controller 2110 may be coupled to one or more airmovement devices for controlling the movement of air through the airchannel.

In some implementations, controller 2110 may provide one or more controlsignals to particle detection module 2130 to generate and adjust thermalgradients in the air stream. For example, thermal gradients in the airstream may be adjusted by adjusting power applied to one or more heaterelements that generate the thermal gradient or by adjusting an airstreamvelocity of the air stream in the air channel.

FIG. 22 shows a block diagram of a method 2200 for analyzing particlesin an air stream. The method 2200 includes applying power to heaterelements positioned on various sides of an air channel encompassing atleast a portion of the air stream, as shown in block 2205. Power may beapplied to one or more pairs of heater elements that may be positionednear a periphery and on opposite sides of the air channel. In someimplementations, the entire length of the air channel in thethermophoretic particle concentrator functions as a heater. In otherimplementations only short portions of the air channel function as aheater. In other implementations, sets or arrays of heater elements maybe employed at certain sections of the air channel. These heaterelements may operate at different temperatures and may be individuallyaddressed in order to provide a high degree of flexibility in thegenerated thermal gradient.

In some implementations, the power (e.g. electrical power) to the heaterelements may be duty cycled (turned on and off) to extend the lifetimeof system components. In many use cases, the time constant associatedwith any significant change in particulate matter concentration is onthe order of tens of seconds to minutes or hours or more. Since airquality measurements may only be needed to be conducted once every fewseconds or few minutes, or every few hours, there may be extendedperiods of time during which sampling of particulate matter may beturned off.

Air may be drawn through the air channel, as shown in block 2210. Thedrawn air may generate the air stream within the air channel. Air may bedrawn through the air channel using any one of a variety of air movementdevices such as a pump, blower, fan, turbine, motorized air intakedevice, bellows pump, membrane pump, peristaltic pump, piston pump,positive-displacement pump, rotary vane pump, Venturi device, airflowmanagement device, or other air drawing means for moving or drawing airthrough the air channel. Drawing air through the air channel may beperformed with a duty cycle corresponding approximately with the dutycycling of the heater elements.

Thermal gradients may be generated within the air channel, as shown inblock 2215. Heat from electrical power applied to the heater elementscombined with airflow profiles and air channel geometries generate oneor more thermal gradients within the air channel, resulting inthermophoretic forces on particles in the air stream directed mainlytowards the interior or center of the air stream.

Particles in the air stream may be forced away from the periphery of theair channel and towards an interior region of the air channel with thethermophoretic force generated by the thermal gradient to concentratethe particles in an interior region of the air stream, as shown in block2220. Aerosol particles introduced into the inlet of the air channel maybe distributed somewhat randomly throughout the cross-sectional area ofthe air stream. Action by the thermophoretic particle concentrator mayreduce the physical cross-section and narrow the distribution of theparticles flowing in the air stream as the air stream and the particlestraverse the particle concentrator through the use of controlled thermalgradients. Particle concentration may be achieved through the use ofopposing thermophoretic forces aligned with respect to one or more axesof the air channel.

The generated thermal gradients are dependent in part on the loss ofheat into the air stream. The air stream in the air channel may exhibita velocity gradient as a function of distance from the channel wall andlength down the channel. Since the amount of heat removed is a functionof the local velocity of air in the air stream, the generated thermalgradients are functionally dependent on the airstream velocity profile.

Particles concentrated in the air stream may be detected, as shown inblock 2225. In some implementations, particles may be detected bydeflecting the particles with generated thermophoretic forces to directparticles in the air stream away from the interior region of the airchannel and towards one or more particle detectors positioned on a wallof the air channel, where the particles may be collected on a surface ofthe particle detector and cause a change in a resonant frequency of theparticle detector in response to the mass loading on the surface. Insome implementations, the change in resonant frequency over a fixed timemay be determined as an indication of the effective mass added onto thesurface of the particle detector. In some implementations of particularbenefit in environments with a large particulate matter concentration,an adaptive cycle may be used that measures the time to depositparticulate matter on a resonant-based particle detector for apredetermined frequency shift. The system may use at least one processorand be under software control so that when the air particle density ishigh, the unit may sample less frequently in order to extend thelifetime of the sensor.

In some implementations, the thermal gradients in either the particleconcentrator or the particle discriminator may be modulated bymodulating the power to the associated heater elements. Modulation ofthe thermal gradients may spread out the deposition of particles on theparticle detectors to avoid non-uniform deposition and to extend thelifetime of the particle detectors.

The detected particles in the air stream may be analyzed, as shown inblock 2230. One or more algorithms may be applied to detect thefrequency shift of the resonant particle detector. The algorithm mayapply calibration coefficients and various model parameters to determinean effective mass of the particles collected on the surface of theparticle detector and to generate an aerosol mass concentration estimatefor the sampled air. In some implementations, the aerosol massconcentration may be estimated for one or more selected particle sizeranges.

FIG. 23 shows a block diagram of a method 2300 for scanning andanalyzing particles. The method 2300 includes concentrating particles inan interior region of an air stream, as shown in block 2305. Theparticles may be concentrated in the interior region of the air streamwith a thermophoretic particle concentrator including at least twoheater elements disposed on opposing walls of the air channel configuredto cooperatively force particles in the air stream towards the interiorregion of the air stream. The thermophoretic particle concentrator maybe positioned in an upstream direction of the air stream with respect tothe heater elements of a thermophoretic particle discriminator. In someimplementations, the heater elements or the temperature profiles may beconfigured to offset the focused particulate matter from an axis ofsymmetry in the air channel. For example, a pair of heater elements maybe positioned on opposite sides of the air channel and one heaterelement may be driven to a higher temperature than the other heaterelement to offset the beam of particles. In some implementations, theelectrical power applied to the opposing pair of heater elements may bemodulated or change with respect to time to dither or raster theposition of focused particulate matter across the cross-section of theair channel.

The concentrated particles may be deflected from the interior region ofthe air stream towards a peripheral wall with one or more heaterelements in the particle discriminator, as shown in block 2310.

Power may be controlled to each of the heater elements in the particlediscriminator to allow scanning of the deflected particles towards theperipheral wall, as shown in block 2315. The temperature of individualheater elements or heater segments may be modulated as a function oftime to achieve a more uniform particle coating. Alternatively, arraysof heaters may be employed in either the thermophoretic concentrator orthe thermophoretic discriminator where individual heaters arealternately dithered or turned on and off to achieve the desireddeposition profile onto the detector surface.

The concentrated particles may be scanned in a lateral or longitudinaldirection along the peripheral wall, as shown in block 2320. To rasterthe particles in the lateral direction perpendicular to the air stream,the temperatures of heater elements in the thermophoretic concentratormay be controlled to displace the focused particle beam in the positiveor minus lateral direction. For example, the temperature of one focusingheater may be higher than a second focusing heater or vice-versa, or thetemperature difference between opposing heater elements may bemodulated. The heater elements of the thermophoretic concentrator mayinclude a single element or an array of heater elements. In the case ofa precipitation heater that is composed of an array of heater elements,rastering in the lateral direction may require activation of individualheater elements or groups of heater elements at different lateralpositions with respect to the center axis of the particle detector.Rastering in the longitudinal direction parallel to the air stream maybe accomplished by modulating the temperature of the precipitationheater as a function of time so that the beam of particulate matter isswept back and forth along the detector surface in the longitudinaldirection. In the case of a precipitation heater composed of an array ofheater elements, heater elements aligned in the longitudinal directionmay be switched on and off so that the thermophoretic gradient and theparticulate matter deposited onto the detector surface is swept back andforth in the longitudinal direction.

The deflected and scanned particles from the interior region of the airstream may be collected on a surface of a particle detector positionedon the peripheral wall, as shown in block 2325. Power to the heaterelements may be controlled to deflect and collect particles in aselected particle size range onto the surface of the particle detector.The deposition of particulate matter via thermophoresis may cause atapered film deposition or island depositions on the surface of theparticle detector. The temperature of one or more segments of theprecipitation heater may be modulated such that the particle depositionfor each particle size is periodically shifted forward and backwardalong the length of the detector surface and laterally across thedetector surface to provide a more uniform coating of particulates andto increase the lifetime of the particle detector.

A system for analyzing particles may include means for concentratingparticles in an interior region of an air stream, means for deflectingthe concentrated particles in the interior region of the air streamtowards a peripheral wall, and means for scanning the concentratedparticles in a lateral direction along the peripheral wall. The systemmay include means for collecting the particles deflected from theinterior region of the air stream on a surface of a particle detectorpositioned on the peripheral wall and means for controlling power toallow scanning of the deflected particles towards the peripheral walland to deflect particles in a selected particle size range onto asurface of a particle detector.

Non-transitory computer-readable medium may store computer-readableprogram code to be executed by at least one processor for analyzingparticles in an air stream including instructions to cause concentratingparticles in an interior region of an air stream, deflecting theconcentrated particles in the interior region of the air stream towardsa peripheral wall, and scanning the concentrated particles in a lateraldirection along the peripheral wall.

Although the various blocks and steps described in the above processflows and methods are intended to be representative, the steps and theorder of the steps may be altered and still remain within the scope,spirit and claims of this disclosure. Variations in the steps and theorder of the steps may be made without loss of generality, such asperforming one step before another or combining two or more steps intoone step.

While various implementations have been described above, it should beunderstood that the implementations have been presented by way ofexample and not limitation. The breadth and scope of the presentdisclosure should not be limited by any of the implementations describedabove but should be defined in accordance with the following claims,subsequently submitted claims, and their equivalents.

1. A system for analyzing particles in an air stream, the systemcomprising: a first heater element configured to deflect particles in aninterior region of the air stream towards a peripheral wall of an airchannel encompassing the air stream; a second heater elementcontrollable to deflect the particles in a first lateral direction alongthe peripheral wall; and a third heater element controllable to deflectthe particles in a second lateral direction along the peripheral wall.2. The system of claim 1, wherein a first thermal gradient in the airchannel, generated by said first heater element, thermophoreticallyforces particles towards the peripheral wall in a directionperpendicular to the air stream.
 3. The system of claim 1, wherein asecond thermal gradient in the air channel generated by the secondheater element and a third thermal gradient in the air channel generatedby the third heater element cooperate to allow thermophoretic forcing ofparticles in either the first lateral direction or the second lateraldirection along the peripheral wall.
 4. The system of claim 1, furthercomprising: a particle detector positioned on the peripheral wall of theair channel, the particle detector configured to collect particlesdeflected from the interior region of the air stream.
 5. The system ofclaim 4, wherein the first heater element, the second heater element,and the third heater element are configured to allow a scanning of atleast some of said deflected particles onto a surface of the particledetector.
 6. The system of claim 5, wherein the scanning spreads theparticles throughout the surface of the particle detector to increaseparticle collection uniformity or to extend a lifetime of the particledetector.
 7. The system of claim 4, wherein a first thermal gradient isselected to deflect particles in a selected particle size range onto asurface of the particle detector.
 8. The system of claim 7, wherein theselected particle size range includes one of a particle size rangebetween about 0.01 microns and 0.1 microns, 0.01 microns and 0.3microns, 0.1 microns and 1.0 microns, 1.0 microns and 2.5 microns, 2.5microns and 10.0 microns, and 10.0 microns and larger.
 9. The system ofclaim 1, further comprising: a thermophoretic particle concentratorincluding a fourth heater element and a fifth heater element, the fourthheater element and the fifth heater element disposed on opposing wallsof the air channel, the fourth heater element and the fifth heaterelement configured to cooperatively force particles in the air streamtowards the interior region of the air stream; wherein thethermophoretic particle concentrator is positioned in an upstreamdirection of the air stream with respect to the first heater element,the second heater element and the third heater element.
 10. The systemof claim 1, further comprising: a controller; wherein the controller iselectrically coupled to the first heater element, the second heaterelement, and the third heater element.
 11. The system of claim 10,wherein the controller is configured to control power to each of thefirst, second and third heater elements to allow scanning of at leastsome of said deflected particles deflected towards the peripheral wall,and to deflect particles in a selected particle size range onto asurface of a particle detector.
 12. The system of claim 1, wherein theparticles are deflected with a thermal gradient generated by acombination of the first, second and third heater elements, and whereinsmaller particles in the air stream are selectively deflected away fromthe interior region and towards a periphery of the air stream at adifferent rate than larger particles in the air stream.
 13. A method ofanalyzing particles, the method comprising: concentrating particles inan interior region of an air stream; deflecting, with a first heaterelement, said concentrated particles in the interior region of the airstream towards a peripheral wall, thus producing deflected particles;and scanning, with a second heater element and a third heater element,at least some of said deflected particles in a lateral direction alongthe peripheral wall.
 14. The method of claim 13, wherein concentratingparticles in the interior region of the air stream comprisesconcentrating the particles with a thermophoretic particle concentratorincluding a fourth heater element and a fifth heater element, the fourthheater element and the fifth heater element disposed on opposing wallsof an air channel, the fourth heater element and the fifth heaterelement configured to cooperatively force particles in the air streamtowards the interior region of the air stream, and wherein thethermophoretic particle concentrator is positioned in an upstreamdirection of the air stream with respect to the first heater element,the second heater element and the third heater element.
 15. The methodof claim 13, further comprising: collecting the particles deflected fromthe interior region of the air stream on a surface of a particledetector positioned on the peripheral wall.
 16. The method of claim 13,further comprising: controlling power to each of the first, second andthird heater elements to allow scanning of at least some of saiddeflected particles deflected towards the peripheral wall, and todeflect particles in a selected particle size range onto a surface of aparticle detector.
 17. A system for analyzing particles, the systemcomprising: means for concentrating particles in an interior region ofan air stream; means for deflecting the concentrated particles in theinterior region of the air stream towards a peripheral wall, thusproducing deflected particles; and means for scanning said deflectedparticles in a lateral direction along the peripheral wall.
 18. Thesystem of claim 17, further comprising: means for collecting theparticles deflected from the interior region of the air stream on asurface of a particle detector positioned on the peripheral wall. 19.The system of claim 17, further comprising: means for controlling powerto allow scanning of said deflected particles deflected towards theperipheral wall and to deflect particles in a selected particle sizerange onto a surface of a particle detector.
 20. A non-transitorycomputer-readable medium storing computer-readable program code to beexecuted by at least one processor for analyzing particles in an airstream, said program code comprising instructions configured to cause:concentrating particles in an interior region of an air stream;deflecting said concentrated particles in the interior region of the airstream towards a peripheral wall, thus producing deflected particles;and scanning said deflected particles in a lateral direction along theperipheral wall.